Running a Well Pump or Sump Pump on a Generator: Safety and Sizing Basics

In a power outage a well pump stops and a sump pump cannot keep the basement dry. A generator can bridge the gap, but only if it is sized and connected correctly. This guide uses a safety page from a Wisconsin utility, We Energies. It is a utility safety summary, not a code.

Starting watts are higher than running watts

The We Energies page warns that a generator must handle starting wattage, which is higher than running wattage. For motors it gives these multipliers on the running wattage: about 2.5 for most motorized items such as furnace fans, and 7 for well pumps, air compressors and air conditioners. It lists running wattages of 600 for a 1/4 HP motor, 800 for 1/3 HP, 1,000 for 1/2 HP and 1,400 for 3/4 HP.

As an illustration only, using the page's own figures: a 1/2 HP motor at 1,000 running watts times 7 gives about 7,000 starting watts. Your pump's nameplate may differ, so read it and ask the pump dealer. Not verified: the real starting load of your specific pump.

Never run it indoors

The page says never to run a generator inside a home, garage, crawlspace, shed or other enclosed area. Run it only in a well-ventilated spot where the exhaust can be vented safely, and install battery-operated or battery back-up carbon monoxide alarms. The page gives no minimum distance from windows or doors, so follow the generator manual and local rules. Not verified: any specific distance.

Do not backfeed the house

Portable generators are not designed to connect to building wiring. Doing so can feed power back onto utility lines, which the page calls life-threatening to utility crews and to others who touch a downed line. The only safe way to connect to building wiring, according to the page, is a transfer switch installed by a licensed electrician. A transfer switch isolates your circuits and removes the need for extension cords.

If you use extension cords

Use only UL-listed, three-prong cords. Plug the appliance into the cord first, then plug the cord into the generator.

A practical plan

Read the pump nameplate: voltage, horsepower and amps.
Ask an electrician whether a 240 V well pump needs a transfer switch and a larger generator than a 120 V sump pump.
Decide which loads must run (pump, refrigerator, heat) and which can wait.
Test the setup before the storm, outdoors and with a CO alarm in the house.

Fueling and grounding rules are not covered by the source, so follow the generator manual. Check local permit rules for a transfer switch.

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General information only. Not electrical, plumbing, legal or code advice. Confirm details with a licensed professional and your local authority.